1. Field of Invention
The present invention relates to an airlock used to continuously feed a material through the airlock while obstructing the free flow of a gas. It particularly relates to such an airlock for use in delivering tobacco or other friable material.
2. Prior Art
In the production of tobacco products, flowing air is often used to transport and/or classify particles of tobacco. Such air is generally driven by an induced pressure differential along the flow path.
At the terminus of such a transport or classification process, it is often desired to separate the pressurized airflow from the tobacco particles. Such separation is generally accomplished today by a rotary airlock. Such an airlock is shown in FIG. 1 and comprises a funnel-shaped feed inlet, a housing having therein a chamber in the shape of a horizontal cylinder, a vaned rotor which coaxially rotates within the cylindrical chamber, and an outlet at the bottom of the chamber. Examples of similar airlocks are referred to in U.S. Pat. Nos. 2,643,768; 2,988,213; 3,043,315; 3,046,998; 3,116,238; 3,362,414; 3,608,716; 3,665,932; 4,166,027; 4,259,032; 4,308,876; and 4,446,876.
There are problems with such rotary airlocks, primarily an undesired degree of breakage of tobacco strands, and secondarily a high degree of abrasive wear to the airlock itself. As each vane sweeps up from the return wall of the cylinder into the tobacco piled in the intake funnel, the vane begins a shearing action between the pushed and pulled portions of the tobacco strands. This shearing action breaks strands.
As the vane edge strikes the transport wall of the chamber, it creates a high-shear point which may cause further strand breakage or which may jam the rotor. As the vane edge sweeps the transport wall there is a grinding action between edge and wall which further damages tobacco and additionally wears both wall and edge, particularly when sand is present. Interaction between driven and falling tobacco, at the discharge, shears and breaks more tobacco.
Some other patents refer to the use of single endless belts in airlocks.
U.S. Pat. No. 540,155, to Dula et al., refers to an endless belt provided with cross pieces which conducts leaves from a box and discharges them into a pneumatic conveyor. The discharge of air past the endless belt is prevented by a close fit between the cross strips and one rounded end of the box and on the other end is prevented by flexible strips lapping on the cross pieces. This single belt system involves relative motion between the stationary flexible strips and the moving leaves.
U.S. Pat. No. 3,091,244, to Molins et al. refers to an air-pervious conveyor, the end of which may be closed by a tyre or a plurality of paddle elements on a rotatable wheel. In this arrangement the contact point between conveyor and closure means exists only at the point where the wheel element contacts the conveyor.
U.S. Pat. No. 3,514,159 to Labbe refers to an endless band forming one wall of a channel. Tobacco moving through the channel forms an airlock. There is relative motion between the non-moving walls of the channel and the tobacco confined thereby.